The majority of amperometric electrochemical biosensors fall within two categories (1) metal-catalyzed electrochemical biosensors or (2) electron-transfer mediator modified electrochemical biosensors. For example, a metal-catalyzed glucose sensor detects the hydrogen peroxide by-product which is produced in a one-to-one ratio from glucose through an enzyme-catalyzed air oxidation process, such as: ##STR1##
U.S. Pat. No. 3,539,455 (1970) by Clark discloses a platinum based glucose sensor useful for self-monitoring of glucose by diabetics. Guilbault and Lubrano (1973) reported amperometric biosensors having an immobilized-enzyme platinum electrodes suitable for glucose sensor applications. Mizutani et al. (1992) reported a platinum/carbon paste (CP) composition with a 1/9 Pt/C ratio suitable for making glucose sensors. U.S. Pat. No. 4,970,145 (1990) to Bennetto et al. discloses a biosensor with a porous enzyme electrode comprising platinized carbon paper having a fluoropolymer binder. These platinum/C based biosensors have sensitivity for detection of glucose only at concentrations of millimolar (mM) glucose with an electric current response of &lt;20 uA/cm.sup.2.mM glucose. Furthermore, the use of a metal or metal/carbon with a high metal content as the working electrode leads to high material costs and has a drawback of loosing hydrogen peroxide due to the metal-catalyzed decomposition of hydrogen peroxide.
An electron-transfer mediator modified electrochemical biosensor relies on a fast electron-transfer mediator, which is typically an outer-sphere metal complex or organic compound, to assist the shuttling of electrons from reduced enzyme molecules to the working electrode. The reduced enzyme results from an enzyme molecule receiving two electrons from a glucose molecule, such as: EQU Glucose+(GOD).sub.oxidized .fwdarw.Glucolactone+(GOD).sub.reduced
Electrochemical biosensors having carbon based working electrodes modified with electron-transfer mediators have been reviewed by Wring and Hart (1992). Biosensors having working electrodes modified with redox polymers have been reviewed by Pishko (1995). These mediator modified biosensors are capable of measuring glucose at mM concentration with an electric current response of &lt;20 uA/cm.sup.2.mM glucose. There remains a need for materials suitable for fabrication of enzyme electrodes with high catalytic activity/current response and low background current noise to expand the capability of biosensors for monitoring biological analytes at the micro-molar (uM) level and to assure a high confidence of detecting low level of analystes in body fluids. One example was given by Tamada, Bohannon and Potts (1995) who reported the iontophoretic extraction of body fluid. The body fluid can then be analyzed in situ for a glucose levels and thus provide a method for non-invasive monitoring of glucose. The glucose concentration in the extracted body fluid is typically in the micro-molar level which produced electric current in the nano-ampere (nA) level, and thus requires a biosensor with low detection limit of glucose determination. A key limiting factor which affects the glucose detection limit is electrochemical signal noise or background current, which may be from electrochemically active impurities or temperature fluctuation etc. It is desirable that a biosensor has low background current which also does not greatly vary with temperature fluctuation.
Furthermore, it is desirable that catalyst materials for the working electrode be low cost and suitable for low cost fabrication of disposable biosensors by conventional printing processes. It is the object of the invention to overcome the deficiencies of current carbon-based materials for working electrodes.